The present invention relates to a flight control system for a rotary-wing aircraft, particularly for a helicopter.
It is known that in a helicopter which has no assisted flight control system such as, for example, an electric flight control device (CDVE) or an automatic pilot, there is a correspondence between, on the one hand, the position of the flight controls, for example the collective pitch lever, the cyclic pitch stick or the rudder, which can be actuated by a pilot of the aircraft and, on the other hand, the axis command applied to the helicopter rotor(s) in response to actuation of at least one of said flight controls. This correspondence allows the pilot to estimate in particular what proportion of control he still has along each axis. This estimate is generally used by the pilot to evaluate his margin for maneuver.
By contrast, when the helicopter is equipped with an assisted flight control system of the aforementioned type, which has an action on helicopter stability, on the decoupling or on the adherence to flight parameters such as attitude or heading, the aforementioned correspondence is no longer present, as specified hereinbelow.
Thus, for example, in the case of a helicopter equipped with an automatic pilot, a jack is generally mounted in the control linkage and receives commands from said automatic pilot, these being added to the commands representative of the position of the flight controls. In consequence, the position of said flight controls no longer corresponds directly to the axis command applied to the rotor(s). This axis command is in fact in this case the result of the sum of the commands representing the position of the flight controls and of the commands generated by said automatic pilot.
A similar situation is encountered in an aircraft equipped with an electrical flight control device (CDVE). In this case, a computer replaces the aforementioned linkage and jack and formulates an overall axis control command including the commands from the pilots and the commands from the assisted control device.
Current evolutions in flight control systems are tending to exacerbate the aforementioned problem because, particularly for safety and flight quality reasons, the assisted flight control systems such as the aforementioned ones are having an increasing influence and benefit, thus requiring an increasing amount of authority. This actually entails being able to generate higher authority than is currently used in conventional equipment.
So, in order to be able to give the aircraft pilot feedback regarding the control margin remaining on each of the flight controls, document FR-2 756 252 discloses a flight control system for a rotary-wing aircraft, particularly for a helicopter, equipped with an assisted flight control system, which, in a simple and precise way, allows the pilot to be at least partially provided with feedback regarding said control margins left on the various flight controls while at the same time acknowledging the flight control assistance afforded by said assisted flight control system.
To achieve this, that known flight control system comprises:
at least one flight control which can be moved between extreme positions by a pilot of said aircraft, first commands being representative of the position of said flight control;
an assisted flight control system generating assistance values representing second commands;
first means for determining control commands for at least one rotor of the aircraft from the sum of said first and second commands, and
a limitation system associated with said assisted flight control system and comprising a limiter which is intended to limit said assistance values according to the position of said flight control, to limited values (transmitted to said first means as second commands) so that, particularly when said flight control is brought near to one of said extreme positions, said limited values are zero and therefore said control commands approximately correspond to said first commands.
Thus, by virtue of this known flight control system:
the pilot can ascertain the control margin remaining for the positions of the flight control, for which this information is important, namely essentially for positions close to said extreme positions; and
the flight control assistance (that is to say the assistance afforded in the form of said second commands) is acknowledged to the greatest possible extent, that is to say particularly when the remaining control margin is sufficient for the pilot not to have precise feedback on it.
However, in the case of a flight control system such as this comprising a system for limiting the authority, a problem is encountered when the boundaries limiting the authority are reached, because at said boundaries, there is an abrupt change from a control law incorporating the flight control assistance (when the second commands are not limited) to a xe2x80x9cpseudo-directxe2x80x9d control law with no stabilization (with second commands limited so that the control commands then correspond exclusively to said first commands increased by the value of said limit on the second commands, which depends on the position of the control) and with a different feel, and vice versa. The transition at said boundaries of the limit of authority is therefore far too abrupt, which in particular leads to problems of flying comfort and accuracy.
The object of the present invention is to overcome these drawbacks. The present invention relates to a flight control system comprising a limitation system which makes it possible to bring about a damped transition at the boundaries of the limit of authority generated by said limitation system.
To this end, according to the invention, said flight control system of the type particularly comprising:
at least one flight control which can be moved by a pilot of said aircraft, first commands being representative of the position of said flight control;
an assisted flight control system generating assistance values representing second commands;
a limitation system associated with said assisted flight control system and comprising a limiter which is intended to limit said assistance values according to the position of said flight control, to first limited values; and
first means for determining said control commands from the sum of said first and second commands,
is noteworthy in that said limitation system additionally comprises:
second means for determining damping values representative of the difference between said assistance values and said first limited values; and
third means for calculating second limited values corresponding to the sum of said first limited values and of said damping values, and for transmitting the second limited values thus calculated as second commands to said first means.
Thus, as far as the assistance afforded by said assisted flight control system and limited by said limiter is concerned, a supplement obtained from the difference between the overall assistance control (assistance values) and said limited assistance control (first limited values) is superposed by virtue of the invention on the limited assistance control (first limited values), which makes it possible to damp the transition in the control commands at the boundaries of the limit of authority generated by said limitation system.
This damping is performed according to the amplitude and, as will be seen below, the dynamics of the assistance (assistance values) determined by the assisted flight control system.
In consequence, the present invention makes it possible, in the medium term (and to within the dynamics of said control supplement), to provide a solution to the problem of providing feedback regarding the proportion of axis control remaining, while at the same time improving the continuity and significance of the assistance, at the boundaries of the limit of authority.
Advantageously, said second means comprise:
a calculation means which calculates the difference between said assistance values and said first limited values; and
a filtering means which filters this difference to remove the low-frequency signals from it so as to obtain said damping values, which allows the dynamic part of the control supplement to be acknowledged.
It will be noted that the time constant of said filtering means determines the significance of the damping of the limit of authority.
As a preference, said limiter is formed in such a way as to limit the assistance values, by continuous limitation functions, generating minimum limitation when said flight control is brought near to its central position, and maximum limitation when said flight control is brought near to one of its extreme positions.
In addition, in order to optimize the efficiency of the present invention, at least some of said limitation functions may advantageously have variable values dependent on the value of parameters, such as the angular velocity of the aircraft, for example, which can be measured thereon.
Moreover:
when the aircraft is equipped with an automatic pilot, said assisted flight control system preferably corresponds to said automatic pilot of the aircraft; and
when the aircraft is fitted with an electrical or optical flight control device, said assisted flight control system and said limitation system are preferably incorporated into said flight control device.